Enteroceptors which respond to transverse stretching and contraction of smooth muscle in dog trachea have been studied by electrophysiological techniques for several years. These receptors continue to respond for the duration of the stimulus and have myelinated axons. They have been estimated to comprise about 45 percent of the pulmonary stretch receptors in the dog and are thought to contribute to the Hering-Breuer inflation reflex and thus to be part of the mechanisms which regulate depth and rate of breathing. Sensory terminals which appear to correspond with these receptors have been located in the part of the trachea wall containing the smooth muscle. At the electron microscope level, they appear quite similar to other unencapsulated mechanoreceptors such as the aortic baroreceptors. The main objective of the proposed work is to further investigate these receptors structurally by using serial sections to make three-dimensional models of them and by examining the ultrastructure of a number of receptors at different points in their course. The orientation of the sensory terminals with respect to muscle and connective tissue will be investigated as part of the reconstructions, and the nature of their attachment of these structures will be elucidated in ultrastructural studies. Similar techniques will be used to investigate "irritant receptors," which are believed to be involved in protective responses and other aspects of the control of breathing.